Remove the operators to convert a legacy pose library (from Blender 2.93
and older) to the current system (introduced in Blender 3.0).
The removal is mostly because the pose markers do not play well with
slotted Actions. The conversion code was never updated for those, and by
now it's easier to remove them than to update the code to the current
data model.
Technically it was still possible to create a legacy pose library (an
Action with pose markers) in current versions of Blender, and then use
this operator to convert that to a modern pose library. I don't know of
anybody doing that, though.
This is part of #146586
Pull Request: https://projects.blender.org/blender/blender/pulls/147061
44913ffb60